The Fischer-Tropsch process can be used for the conversion of synthesis gas into liquid and/or solid hydrocarbons. The synthesis gas may be obtained from hydrocarbonaceous feedstock in a process wherein the feedstock, e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass, is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds and water in the actual Fischer-Tropsch process. The obtained paraffinic compounds range from methane to high molecular weight molecules. The obtained high molecular weight molecules can comprise up to 200 carbon atoms, or, under particular circumstances, even more carbon atoms. Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
Catalysts used in the Fischer-Tropsch synthesis often comprise a carrier-based support material and one or more metals from Group 8-10 of the Periodic Table of Elements, especially from the cobalt or iron groups, optionally in combination with one or more metal oxides and/or metals as promoters selected from zirconium, titanium, chromium, vanadium and manganese, especially manganese. Such catalysts are known in the art and have been described for example, in the specifications of WO 9700231A and U.S. Pat. No. 4,595,703.
The hydrocarbon product stream obtained after the Fischer-Tropsch synthesis comprises mainly paraffinic compounds ranging from methane to high molecular weight molecules. Of this range of products the lighter part (i.e. methane (C1) to butane (C4)) are the least desired products and the heavier part the more desired part of the product stream. Most valued are the hydrocarbons ranging from C5 to C40 (C indicating the carbon chain length). The lighter part of the product stream is normally recovered from the product stream as tail gas and can be reused upstream of the Fischer-Tropsch process (for example in the synthesis gas production).
There are several ways known to improve the yield of the intermediate part of the product stream obtained from a Fischer-Tropsch reaction. It is possible to change the catalyst formulation and select a catalyst with an improved yield to this desired part of the product stream. Once the catalyst has been selected the distribution is fixed for a large extent. Moreover, even with the same catalyst a relative small change is possible by varying the concentration of CO, H2 and inert in the gaseous stream towards the reactor. Finally it is possible to change the operating temperature of the catalyst. There is a continuing desire in the art to improve the Fischer-Tropsch process, especially to tune the product distribution for a given catalyst during its use.